The present embodiments relate to a method and a control sequence determination device for determining a magnetic resonance system activation sequence.
In a magnetic resonance system, the body to be examined may be exposed to a relatively high basic field magnetic field, of 3 or 7 Tesla, for example, with the aid of a basic field magnet system. A magnetic field gradient is also applied with the aid of a gradient system. High-frequency excitation signals (HF signals) are transmitted by way of a high-frequency transmit system using suitable antenna devices to tip the nuclear spin of certain atoms that have been excited in a resonant manner by the high-frequency field with spatial resolution through a defined flip angle in relation to the magnetic field lines of the basic magnetic field. The high-frequency excitation and the resulting flip angle distribution is also referred to below as nuclear magnetization or abbreviated to magnetization. During relaxation of the nuclear spin, high-frequency signals (e.g., magnetic resonance signals) are emitted. The high-frequency signals are received using suitable receive antennas and are further processed. Raw data acquired by the receive antennas may be used to reconstruct desired image data. Transmission of the high-frequency signals for nuclear spin magnetization takes place using a body coil or local coils present on the patient or participant. A structure of the body coil may be a birdcage antenna consisting of a number of transmit rods disposed around a patient chamber of the tomography system, in which a patient is present during the examination, running parallel to the longitudinal axis. End faces of the antenna rods are connected respectively in a capacitive manner in a ring.
The body antennas may be operated in a “homogeneous mode” (e.g., a “CP mode”). A single temporal HF signal is emitted to all components of the transmit antenna (e.g., all the transmit rods of the birdcage antenna). In this process, the pulses may be transferred to the individual components in a phase-shifted manner with a displacement tailored to the geometry of the transmit coil. For example, in the case of a birdcage antenna with 16 rods, the rods may each be shifted through 22.5° phase displacement with the same HF magnitude signal. The homogeneous excitation results in a global high-frequency exposure of the patient, which is limited according to rules, as too high a high-frequency exposure may harm the patient. The high-frequency exposure of the patient may be calculated beforehand when planning the high-frequency pulses to be emitted, and the high-frequency pulses are selected so that a certain limit is not reached. HF exposure in the following may be a physiological exposure induced by the HF irradiation and not the HF energy introduced. A typical measure of high-frequency exposure is a specific absorption rate (SAR) value that indicates in watts/kg the biological exposure acting on the patient due to a certain high-frequency pulse output. A standard limit of 4 watts/kg at a “first level” according to the IEC standard applies, for example, for the global SAR or HF exposure of the patient. In addition to the prior planning, the SAR exposure of the patient is also monitored continuously on the magnetic resonance system during the examination using suitable safety devices, and a measurement is changed or terminated if the SAR value is above the specified standards. The most precise prior planning may avoid interrupting a measurement, as interrupting the measurement necessitates a new measurement.
With more recent magnetic resonance systems, individual HF signals tailored for imaging purposes may be assigned to the individual transmit channels (e.g., the individual rods of the birdcage antenna). A multichannel pulse train that consists of a plurality of individual high-frequency pulse trains that may be emitted in a parallel manner by way of the different independent high-frequency transmit channels is emitted. The multichannel pulse train (e.g., a “pTX pulse” due to the parallel emission of the individual pulses) may be used, for example, as an excitation, refocusing and/or inversion pulse.
The multichannel pulse trains may be generated beforehand for a certain planned measurement. In an optimization method, the individual HF pulse trains (e.g., the HF trajectories) are determined for the individual transmit channels over time as a function of a “transmit k space gradient trajectory” that may be predefined by a measurement protocol. The “transmit k space gradient trajectory” (referred to as “k space gradient trajectory” or “gradient trajectory”) refers to the places in the k space that may be started up by setting the individual gradients at certain times (e.g., by gradient pulse trains (with appropriate x, y and z gradient pulses) to be emitted in a coordinated manner as appropriate for the HF pulse trains). The k space is the spatial frequency space, and the gradient trajectory in the k space describes the path in the k space traveled in time during emission of an HF pulse or the parallel pulses by corresponding switching of the gradient pulses. By setting the gradient trajectory in the k space (e.g., by setting the appropriate gradient trajectory applied parallel to the multichannel pulse train), the spatial frequencies, at which certain HF energies are deposited, may be determined.
To plan the HF pulse sequence, a user predefines a target magnetization (e.g., a desired flip angle distribution with spatial resolution) that is used within the target function as a setpoint value. The appropriate HF pulse sequence for the individual channels is calculated in the optimization program, so that target magnetization is achieved. A method for designing the multichannel pulse trains in parallel excitation methods is described, for example, in W. Grishom et al., “Spatial Domain Method for the Design of RF Pulses in Multicoil Parallel Excitation,” Mag. Res. Med. 56, pp. 620-629, 2006.
For a certain measurement, the different multichannel pulse trains, the gradient pulse trains associated with the respective activation sequence and further control defaults are defined in a measurement protocol. The measurement protocol is produced beforehand and may be called up for the measurement from a memory, for example, and optionally changed by the operator. During the measurement, the magnetic resonance system is controlled fully automatically on the basis of the measurement protocol, with the control facility of the magnetic resonance system reading commands out of the measurement protocol and processing the commands.
During the emission of the multichannel pulse trains, homogeneous excitation may be replaced with an excitation of any form in the measurement space and also in the patient. To estimate the maximum high-frequency exposure, every possible high-frequency superimposition may be examined. The high-frequency superimpositions may be examined, for example, using a patient model incorporating tissue-specific attributes such as, for example, conductivity, dielectricity and/or density in a simulation. From previous simulations, “hot spots” may form in the high-frequency field in the patient. The high-frequency exposure may be many times the values known from homogeneous excitation at the hot spots. The resulting high-frequency limitations are unacceptable when performing clinical imaging, as if the hot spots are taken into account, the overall transmit power may be too low to produce acceptable images. When planning the multichannel pulse trains, the local high-frequency exposure may not be too great, but the overall HF transmit power should not be reduced unnecessarily, thereby adversely affecting image quality.